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Pinpoint Test B: DTC P0645

WARNING: This page is about a different car, the 2010 Lincoln Town Car. However, it is still accessible from the selected car via links, so may be relevant.
  1. B1 CHECK THE VOLTAGE TO THE A/C CLUTCH RELAY 
    • Key in OFF position.
    • Disconnect: A/C Clutch Relay
    • Key in ON position.
      • Fig 1: Measuring Voltage
      GF0047605Courtesy of FORD MOTOR CO.
    • Measure the voltage between ground and the A/C clutch relay socket pin 2, circuit 391 (RD/YE).
    • Is the voltage greater than 10 volts? 
    1. YES  : CARRY OUT the A/C Clutch Relay component test in COMPONENT TESTING article. If the relay tests OK, go to  B2.
    2. NO  : VERIFY Battery Junction Box (BJB) fuse 21 (15A) is OK. If OK, REPAIR circuit 391 (RD/YE) for an open. If not OK, REFER to the SYSTEM WIRING DIAGRAMS article to identify the possible causes of the circuit short. CLEAR the DTCs. REPEAT the self-test. TEST the system for normal operation.
  2. B2 CHECK CIRCUIT 73 (OG/LB) FOR A SHORT TO VOLTAGE 
    • Key in OFF position.
    • Disconnect: PCM C175b
    • Key in ON position.
      • Fig 2: Measuring Voltage
      GF0047606Courtesy of FORD MOTOR CO.
    • Measure the voltage between ground and PCM C175b-14, circuit 73 (OG/LB), harness side.
    • Is any voltage present? 
    1. YES  : REPAIR circuit 73 (OG/LB) for a short to voltage. CLEAR the DTCs. REPEAT the self-test. TEST the system for normal operation.
    2. NO  : Go to  B3.
  3. B3 CHECK CIRCUIT 73 (OG/LB) FOR A SHORT TO GROUND 
    • Key in OFF position.
      • Fig 3: Measuring Resistance
      GF0047607Courtesy of FORD MOTOR CO.
    • Measure the resistance between ground and PCM C175b-14, circuit 73 (OG/LB), harness side.
    • Is the resistance greater than 10,000 ohms? 
    1. YES  : Go to  B4.
    2. NO  : REPAIR circuit 73 (OG/LB) for a short to ground. CLEAR the DTCs. REPEAT the self-test. TEST the system for normal operation.
  4. B4 CHECK CIRCUIT 73 (OG/LB) FOR AN OPEN 
      Fig 4: Measuring Resistance
      GF0047608Courtesy of FORD MOTOR CO.
    • Measure the resistance between A/C clutch relay socket, circuit 73 (OG/LB) and PCM C175b-14, circuit 73 (OG/LB), harness side.
    • Is the resistance less than 5 ohms? 
    1. YES  : Go to  B5.
    2. NO  : REPAIR circuit 321 (GY/WH) for an open. CLEAR the DTCs. REPEAT the self-test. TEST the system for normal operation.
  5. B5 CHECK THE PCM CONNECTION 
    • Clear the DTCs.
    • Disconnect all the PCM connectors.
    • Check for:
      • corrosion.
      • pushed-out pins.
      • incorrectly seated connector.
    • Connect and correctly seat all the PCM connectors.
    • Operate the system.
    • Does the concern return? 
    1. YES  : INSTALL a new PCM. TEST the system for normal operation.
    2. NO  : The system is operating correctly at this time. The concern may have been caused by a loose or corroded connector.
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